It is important to the function of an elevator that its load be known at all times--whether to determine if the car is overloaded so that the elevator brake is not released while under that load or to determine which floor should be serviced based of the load in the car. Several types of elevator load weighing systems have been used.
Elevator load weighing systems may be divided into two groups: those that place a sensor beneath the cab platform and those that do not.
Several systems have been designed with the load weigher beneath the car. Some measure the deformation of resilient pads beneath the car platform, U.S. Pat. No. 4,078,623. A disadvantage of the deformation systems is the inaccuracy introduced by hysteresis in the elastomeric material used in the pads. U.S. Pat. No. 4,899,852 discloses an elevator car disposed in a frame which moves on rails through a hoistway; a pendulum mount is used to mount the car in the frame so that the car is free to swing from four suspension rods within the frame in pendulum fashion. Two of the four suspension rods extend through a support pad and a rubber pad and a second support pad. Below two corners of the car, a load cell separates the support pads. This device involves deflection in the load cell in response to weight on the cab floor. There is no positive connection between the load cell and the support pads. The two support pads sandwich the rubber pad. There are several problems. First, there is signal drift due to hardening of the rubber pads. Typically, the platform is supported at some places by wooden blocks and rubber pads and at other places by load cells supported by rubber pads. But the hardness of the rubber pads increases from the time the elevator is installed. And, the hardness of different rubber pads increases at different rates, such that the load evenly distributed on a cab platform is not carried equally by different pads. In addition, accurate load measurement requires that the load cells and rubber pads should be under uniform pressure; achieving this is time consuming and difficult. Further, the load weight signal measurement typically depends on a linear relationship, over the range of weights to be measured, between deflection of a load cell and a load of, for example, 0 to 3000 pounds. On installation, the relationship may be linear but because of the aging of the rubber pads, it becomes nonlinear for a portion of the range of weights to be measured. The result is an erroneous load measurement. When the error in the load measurement becomes excessive the use of the load weigher is undermined almost totally. For example, if the average elevator passenger weighs 150 pounds and the range is 0 to 3000 pounds, the resolution error may be greater than 5% so that it cannot be determined whether or not one person is in the car. Frequently, adolescents will get on an elevator and press all the buttons. Anti-nuisance software estimates the number of people in a car by assuming an average weight per person. It then determines the number of car calls and if the number of car calls exceeds the number of people estimated, it cancels those car calls. If a load weigher cannot distinguish the weight of an average person, this software cannot operate. A further problem with platform load weighing devices is that, even if the load cells are initially installed to equally share the load, different measurements are later obtained depending on the location of the load on the platform because of the hardening of the rubber pads.
It is desirable to weigh the load of the car under any load distribution. U.S. Pat. No. 4,793,442 discloses load cells between the top and middle and then middle and bottom plates, one load cell at each corner of an equilateral triangle, the top plate being connected to the crosshead. Bolts extend from the hoistway ropes through the three plates. This solution involves an unnecessary number of load cells and leads to rubbing between the bolts and the plates. The load weighing teaching in U.S. Pat. No. 4,899,852 has too many load cells to make it useful in connection with U.S. Pat. No. 4,793,442.